Diffusivities in Binary Mixtures of [AMIM][NTf₂] Ionic Liquids with the Dissolved Gases H₂, He, N₂, CO, CO₂, or Kr Close to Infinite Dilution

Abstract

Ionic liquids (ILs) are interesting working fluids in many areas of chemical and energy engineering such as gas separation, catalysis, or energy storage. For the optimum design of related processes and apparatuses, the diffusion coefficient of gases dissolved in ILs is necessary, yet is often only known for a few selected gases. In this work, the thermal and mutual diffusivities in binary mixtures consisting of the gases hydrogen, helium, nitrogen, carbon monoxide, carbon dioxide, or krypton dissolved in the homologous series of the ILswere investigated by dynamic light scattering (DLS) and equilibrium molecular dynamics (EMD) simulations for temperatures between (298 and 423) K at conditions close to infinite dilution of the dissolved gas. Here, the Fick diffusion coefficient determined by DLS can be compared with the self-diffusion coefficient of the dissolved gases predicted by EMD simulations. For the latter, selected force fields (FFs) for the ILs available in literature were tested, for which the most suitable FF was modified by scaling the partial charges of all atoms with an IL-specific factor using experimental data for density, viscosity, and the self-diffusion coefficient of the pure ILs. On the basis of the modified FF, agreement of the calculated gas self-diffusion coefficients and the experimental Fick diffusion coefficients with an average absolute relative deviation of 12% was found. The Fick diffusion coefficient increases with decreasing molecular weight of the dissolved gas, with the exception of hydrogen and helium, which show an inverse behavior. In contrast to the trends observed for binary mixtures of the above gases dissolved in n-alkanes or 1alcohols of varying alkyl chain length, the diffusion coefficients of the mixtures with the different ILs investigated in this study were found to be not significantly affected by the solvent viscosity.

Diffusivities in Binary Mixtures of n-Hexane or 1-Hexanol with Dissolved CH4, Ne, Kr, R143a, SF6, or R236fa Close to Infinite Dilution

“…The present work provides a link to previous investigations ,,,, studying the Fick diffusion coefficient D 11 as well as the structure–property relationships in the liquid phase of binary mixtures consisting of a liquid and a dissolved gas close to infinite dilution. As solvents, nonpolar n -alkanes, polar 1-alcohols, or bis­(trifluoromethylsulfonyl)­imide ([NTf 2 ] − )-based ionic liquids (ILs) with varying carbon chain lengths were investigated.…”

“…The results for the n -alkane- and 1-alcohol-based systems obtained in the previous work served for the development of a semiempirical model for the prediction of D 11 . , This model requires only pure-component properties of the solute and solvent and was able to predict D 11 of the studied systems and a set of similar systems from the literature with a typical deviation of 10% . The studies regarding the [NTf 2 ] − -based ILs have furthermore shown that in such highly viscous systems, D 11 is nearly independent of the viscosity of the IL solvents at a given temperature . Aiming to study the influence of the larger molar weight and dipole moment of the solutes on D 11 close to infinite dilution, investigations in the present study are extended to binary mixtures consisting of n -hexane (C 6 H 14 ) or 1-hexanol (C 6 H 14 O) with the dissolved gases methane (CH 4 ), neon (Ne), Kr, 1,1,1-trifluoroethane (R143a), sulfur hexafluoride (SF 6 ), or 1,1,1,3,3,3-hexafluoropropane (R236fa).…”

“…Simulations were performed with the open-source simulation software package GROMACS version 5.1.2. To begin, 1000 solvent molecules and 31 solute molecules are randomly placed in a cubic simulation box for a constant mole fraction of the dissolved gas, x 1 = 0.0301, which can be considered to be close to infinite dilution. ,, Here, for 1000 solvent molecules, a solute mole fraction of about 3% was found to be large enough to access the solute self-diffusion coefficient with expanded statistical uncertainties below 10% ( k = 2). Boundary conditions of the box are periodic to mimic the bulk behavior of the compressed liquid phase, with the typical box length ranging from 5.9 to 7.0 nm.…”

“…Dynamic light scattering (DLS) has been proven to be an excellent tool for the simultaneous and accurate determination of a and D 11 in liquids with dissolved gases under various thermodynamic conditions. − In contrast to conventional techniques, DLS does not require calibration and is applied at the macroscopic thermodynamic equilibrium. Therefore, no macroscopic gradient, which may lead to a convective/advective mass transport besides the molecular one, must be applied.…”

“…Therefore, the simultaneous employment of EMD simulations with experimental work offers a direct evaluation and justification of simulation results, while the simulation results help us to get a better understanding of the experimental results. For the investigations close to infinite dilution, D 11 measured by DLS can directly be compared to the self-diffusion coefficient of the dissolved gas D 1 . ,,, …”